The present invention relates broadly to an engine test apparatus, and in particular to a rotor blade test apparatus.
In a jet engine, the rotating blades of the inducer rotor undergo excititations in a very wide range of frequencies. Because of this, the blades may go into resonance, and the amplitude of oscillation may reach destructive values. Knowing the exact value of these resonance frequencies in the various modes make it possible to minimize the harmful effects by applying appropriate remedies.
In the prior art, various techniques to excite a rigid member by means of a sound generator have been utilized. The blade's resonance is in general detected at its tip by means of a displacement transducer which is therein mounted. This transducer furnishes the frequency and amplitude of the blade's resonance. The maximum is sought by varying the frequency of the excitation signal and by correlatively measuring its amplitude in the displacement pick-up. The disadvantages in the use of such methods are the relative slowness and the resulting poor precision. Indeed, in the vicinity of the resonance frequency, the amplitude curve varies very little, and finding the maximum proves to be a delicate procedure if good precision is desired.
In general, a rigid member will ring when struck on its surface. The resulting sound wave has a characteristic or natural frequency for the specimen, and diminishes progressively in amplitude with time, the rate of diminishing with time being also characteristic of the rigid member. The characteristic resonant frequency and the damping factor vary observably with variations in the material and structural condition of the rigid member. This principle is known, and has been utilized in nondestructive examination of rigid members. For example, a rigid member that has been tapped on the surface, may be examined acoustically by ear to determine the contents and structural condition. For instance, this method has been applied to examine the degree of bond between a bearing metal and its linings, or for the existence of blowholes in the castings. However, this method is only useable by those who have been specially trained to hear such effects, and the resulting data is not always reliable by reason of individual variability.